Riser pipe and guide line tensioning apparatus

ABSTRACT

An offshore floating well drilling rig has tensioning apparatus including a power cylinder and an accumulator cylinder structurally integrated and connected with the riser pipe and guide lines to maintain tension thereon as the platform moves with changes in the water level.

[ July 29, 1975 12,111 gtates amt 1 1 Butler [54] RISER PIPE AND GUIDE LKNE TENSIONKNG 3,158,208 11/1964 Kamrnerer 175/5 Parks APPARATUS Ryan.....,........ Bates et a1.

[75] Bryan V. Butler, Ventura, Calif.

Vetco Offshore lndustries, 1nc., Ventura, Calif.

Sept. 12, 1973 Inventor:

[73] Assignee:

Primary Examiner-r obert J. Spar Assistant Examiner-Kenneth Noland Attorney, Agent, or FirmBernard Kriegel [22] Filed:

Appl. N0.: 396,493

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References Cited UNITED STATES PATENTS 3,158,206 Kammerer.....................,..,. 254/172 21 Claims, 11 Drawing Figures PATENTED JUL 2 91975 SHEET PATENTEU 7 SHEET 2 5 .897, 045

PATENTEUJULZQIQYS SHEET SHEET lira-.66.

I RISER PIPE AND GUIDE LINE TENSIONING APPARATUS l In the drilling of wells into the earth at offshore locations, from floating vessels or platforms which rise and fall relative to the ocean floor with the changes in elevation of the water, due to tide changes and waves, in many instances it has become the practice to locate equipment, such as the usual blowout preventors used during the drilling operation on the ocean floor, and the drilling operations are conducted through marine riser or pipe means which extend upwardly through the water from the bottom of the water or ocean floor. In such operations, equipment is raised and lowered under the control of guide lines which extend upwardly from the ocean floor between a guide base located on the ocean floor and the platform. It has been the practice, furthermore, to employ tensioning devices including cables oflines connected to the guide lines and riser pipe and extending to the platform or floating vessel.

Such riser tensioning lines and guide lines have been tensioned in various manners, including the use of counterweights, which have practical limitations as the depth of water increases, dynamic tensioning devices, in the form of compressed air and/or air and oil for storing energy to apply tension to the riser tensioning lines and guide lines. In general, such dynamic tensioning devices have involved the use of cylinders from which a piston rod extends, the piston rod having a sheave engaged by the line to be tensioned, and the forces applied to the piston rod have placed the piston rod in compression, the fluid within the cylinder being thereby compressed into an accumulator. The cylinder and the accumulator have been supported on the platform on bulky support structures. As a consequence, such tensioning devices have been bulky and subject to excessive wear.

The present invention provides tensioning means for riser pipes and guide lines which is less bulky and less subject to wear than those heretofore available, and wherein the accumulator cylinders constitute structural members which support the power cylinder, thereby eliminating the bulky structural supports, and the connection between the accumulator cylinders and the power cylinder is self-aligning, reducing wear.

More particularly, the arrangement of the accumulator cylinders and the power cylinder and piston is such that the line tension applies force to the power cylinder in a manner causing the piston rod to be in tension, and the pressurizing of the fluid in the accumulators by the piston, places the accumulators under an internal pressure which, is more or less offset by the compressive force applied to the accumulator cylinders by the power cylinder, due to the structural relation of the cylinders, wherein the power cylinder is supported by the accumulator cylinders themselves.

The invention may be availed of in forms wherein the compressed air which applies tension to the lines is separated from a liquid, such as oil, in the power cylinder by a floating barrier or piston, where the disposition of the cylinder structure is such that the liquid is disposed above the pressurized air, or, alternatively, the structure may be inverted so that the pressurized air is above the liquid or oil.

In accordance with the present invention, a derrick or platform is equiped with four tensioning devices for maintaining the guide lines under uniform tension between the platform and a subsurface guide base at the ocean floor, and four tensioning devices are also provided for maintaining tension on lines connected to the riser pipe which extends upwardly from the ocean floor.

This invention possesses many other advantages, and has other purposes which may be made more clearly apparent from a consideration of the forms in which it may be embodied. These forms are shown in the drawings accompanying and forming part of the present specification. They will now be described in detail, for the purpose of illustrating the general principals of the invention; but it is to be understood that such detailed descriptions are not to be taken in a limiting sense.

Referring to the drawings:

FIG. 1 is a perspective view showing an offshore well drilling platform, and the subsurface guide base at the ocean floor, with a marine riser extending upwardly to the platform, the platform being equiped with tensioning devices made in accordance with the invention for maintaining the tension on guide lines and riser tensioning lines connected with the guide base and riser pipe;

FIGS. 2a and 2b, together, constitute a side elevation of one of the tensioning devices of FIG. 1, taken substantially on the line 2-2 of FIG. 1, FIG. 21) being a downward continuation of FIG. 2a;

FIGS. 3a and 3b, together, constitute a side elevation of the tensioning apparatus of FIG. 2a and 2b, with parts broken away as taken substantially on the line 3-3 of FIGS. 2a and 2b, FIG. 3b being a downward continuation of 3a;

FIG. 4 is a fragmentary detail in vertical section and on an enlarged scale, as taken on the line 44 of FIG. 3a, showing the connection between the power piston rod and the frame structure of the accumulator cylinders;

FIGS. 5a and 5b, together, constitute a side elevation of another of the tensioning devices of FIG. 1, taken substantially on the line 5-5 of FIG. 1, FIG. 5b being a downward continuation of FIG. 5a;

FIG. 6a and 6b, together, constitute a fragmentary detail in vertical section and on an enlarged scale, as taken on the line 6-6 of FIGS. 5a and 5b, showing the connection between the power piston rod and the frame structure of the accumulator cylinders; and

FIG. 7 is a fragmentary detail view in vertical section, as taken on theline 77 of FIG. 5a.

As seen in the drawings, referring first to FIG. 1, a well drilling platform P of the type adapted to float in or on a body of water, such as the ocean, has the usual derrick D from which well drilling operations are conducted into the earth through the ocean floor F. On the ocean floor is a guide base G, and a marine riser R extends upwardly, so that drilling and completion operations can be conducted from the platform P through the riser R. In order to enable the platform to rise and fall with changes in the elevation of the water, caused by tide and wave action, the riser R consists of a fixed lower pipe portion R1 and a telescopic upper riser pipe portion R2. The guide base G has connected thereto four guide lines GL which extend upwardly to the platform P to four similar guide line tensioning devices GT made in accordance with the invention, and to be hereinafter more fully described, whereby the guide lines GL are maintained under tension, notwithstanding the rise and fall of the platform. correspondingly, in order to maintain the lower riser pipe R1 in tension, a number of riser tensioning lines RL are connected to the upper end of the riser pipe R1 and extend to the platform for connection with four riser tensioning devices RT, made in accordance with the invention, and adapted, as will later be described, to maintain the riser pipe R1 under tension, notwithstanding the rise and fall of the platform P.

The tensioning devices GT and RT are of the type employing air as a medium for applying tension to the respective lines GL and RL. Accordingly, as diagramatically illustrated, air is supplied to the respective tensioning devices under the control of a suitable control panel via suitable piping 11, from a suitable source of air which is shown as comprising compressor means C and a standby reservoir means C1, from either of which air is adapted to be supplied to the control panel via a conduit 12, the air passing through well known means 13 for drying the air.

Referring to FIGS. 2a through 4, a typical riser tensioning device RT, made in accordance with the invention, is shown mounted on a structural member or support 15 forming part of the platform by means of a lower mounting bracket or base 16 and an upper bracket or support clamp 17, the base 16 and clamp 17 being connected to the structure 15 by suitable fastenings 16a and 17a.

The lower mounting bracket or base supports a revolvable cathead 18 about which a number of runs 18a of the riser tensioning line RL extend, the line RL then being reeved about a stationary block 19, the cathead 18 and stationary block 19 being rotatable about a supporting or bearing pin 20 carried by frame members 21 integral with the base support 16, and the line RL hav ing a so-called dead line or end anchored in a suitable fashion to a stationary point. Runs 22 of the line RL are reeved over an upper, traveling block 23 which comprises a frame structure 24 rotatably supporting a line sheave 25. As will be later described, the traveling block frame 24 is connected at 26 with the tensioning cylinder means TC for maintaining tension on the riser tensioning line RL as the line is pulled and let out by the rise and fall of the platform P to cause the traveling block 23 to move vertically towards and away from the stationary block 19.

The frame 24 of the traveling block structure 23 is, as previously indicated, connected at 26 to the tensioning cylinder or power cylinder means TC, as best seen in FIG. 4. As shown, the frame structure 24 has side plates 24a and 24b connected to the piston rod 30 of the power cylinder means TC; by a split block assembly including block sections 31 and 32, respectively connected to the plates 24a and 24b by fastenings 33, 34 and block sections 35 and 36 interconnected by fasteners 37 for engagement about the lower end of the rod 30. The rod 30 has a groove 38 which receives companion internal flange portions 39 and 40 on the block 31 and 32 and flange portions 41 and 42 on the block sections 35 and 36 36 to connect the rod 30 to the frame 24, while allowing the lower end of the rod 30 to be of a size adapted to accommodate a sealing sleeve 43 over the end of the rod 30.

This sealing sleeve 43 is disposed in a bore 44 in the lower end of the tubular body or cylinder 45 of the power cylinder means TC. At its lower end, the sleeve 43 has an end flange 46 fastened on the body 45 by screws 47, a seal ring 48 preventing leakage between the sleeve 43 and the body 45. In the sleeve 43 are axially spaced ring seals 49 and bearing rings 50, whereby, the rod 30 is centralized in the body 45 and leakage along the rod 30 is prevented.

The structural support for the tensioning or power cylinder TC comprises the accumulator cylinder means AC, comprising cylinder bodies 51 and 52 mounted on base flanges and sealing heads 53 and 54 which, in turn, are fastened at 54a to the mounting bracket or base 16. These cylinders 51 and 52 are clamped by collars or yokes 50a and 51a to the upper support clamp 17, so that the cylinders 51 and 52 are columnar supports affixed to the supporting structure 15 previously referred to. By means enabling automatic alignment of the rod 30 with the line of force applied to the frame 24 of the traveling block assembly 23, the tensioning cylinder TC is supported at the upper ends of the accumulator cylinders 51 and 52.

In this connection, the connecting means 60, in this form, comprises upstanding brackets 61 and 62 on cylinder heads or caps 63 and 64 of the accumulator cylinders 51 and 52 to which a horizontal yoke or collar 65 is pivotally connected at 66, at opposite sides of the cylinder body 45. The yoke 65 supports pillow blocks 67 and 68 in which are pivotally supported trunnions 69 and 70 of a collar 71 also disposed about the cylinder body 45. Thus, the tensioning cylinder body 45 can partake of universal or self-aligning movement about the pivots 66 and trunnions 69 and 70 for alignment with the line of force applied by the tensioning line RL for the riser R1.

At the upper end of the cylinder body 45 of the tensioning cylinder TC is a closure or cylinder head 75 and at its lower end is the end flange 46. in the body 45 is a piston 77 having sealing rings 78 and connected to the rod 30. Below the piston 77, the body 45 has a fluid inlet fitting 79 connected by a conduit 80 with a branch conduit which leads to the upper cylinder heads 63 and 64 of the accumulator cylinder 51 and 52 to establish fluid communication between the tension cylinder body 45, below the piston 77 and the upper ends of the accumulator cylinders 51 and 52. As previously indicated, air supplied from the control panel 10 is admitted via inlet fittings 91 and 92 adjacent to the lower heads 53 and 54 of the accumulator cylinders 51 and 52, at a pressure determined at the control panel, whereby to provide a fluid pressure force acting on the piston 77 tending to urge the piston and rod upwardly to apply tension to the riser tensioning line RL. As the tension on the riser line RL is increased, pulling the traveling block assembly 23 toward the stationary block, the air pressure within the system will provide resistance which maintains the tension as the air is compressed, but without significantly increasing the air pressure. When the tension on the tensioning line RL is relaxed, the stored energy in the accumulator cylinders 51 and 52 will urge the tensioning cylinder piston 77 upwardly, thereby maintaining substantially a continuous and constant pressure on the riser line RL.

As illustrated, the system is hydraulic and pneumatic and free pistons 93 and 94 having seal rings 95 and 96, respectively, are shiftable in the accumulator cylinders 51 and 52 and form a barrier separating a body of liquid 97 in the respective accumulator cylinders 51 and 52 and in the tensioning cylinder body 45 from the air or other gas in the accumulator cylinders 51 and 52 below the pistons 93 and 94.

This hydraulic and pneumatic system as just described requires the pistons 93 and 94 because the liquid body ,97 in the accumulator cylinders 51 and 52 is above the air in the respective accumulator cylinders. Such pistons 93 and 94 are not necessary when a wholly pneumatic tension is utilized or when, as shown in the embodiments of FIG. 5a through 7, the body of liquid is below the air in the accumulator cylinders.

In the last-mentioned embodiment, the tensioning means are illustratively the guide line tensioning means GT. i

Here again, however, the line to be tensioned, i.e., the guide line GL, leads from the guide base G over the illustrated guide pulley to the stationary block 119 about which it is reeved and extends upwardly in runs 122 above the traveling block assembly 123. As in the previous embodiment, the traveling block assembly 123 is connected at 126 to the tensioning cylinder rod 130 which has at its upper end the piston 177 reciprocable in the tensioning cylinder body 145.

Also, as in the previously described embodiment, the accumulator cylinders 151 and 152 are supported on the statinary base support 116 and extend upwardly as structural columns, being connected at 160 with the tensioning cylinder body 145, in the manner shown in FIG. 7, so as to enable self-alignment of the tensioning cylinder means TC with the line of force applied by the guide line GL. In this embodiment, a support plate 171 fixed tothe upper ends of the cylinders 151 and 152 as by welding at 171a has a generally conical seat l7lb providing at least at circumferentially spaced locations, inclined downwardly and inwardly inclined support surfaces engageable by a collar 171C welded at 171d or otherwise affixed to the tensioning cylinder body 145 and providing a radially outwardly and upwardly extending generally spherical surface -l71e adapted to seat upon the surface 171b in the manner ofa universal or ball socket support. Suitable retaining lugs l71fmay be bolted to the member 171 and provided with lugs 171g overlying the collar 171C to provide retaining means.

In this embodiment, air is supplied from the control panel via the pipe 11 to the fitting 179 and crossover line 119a, to both of the cylinders and 152. In the illustrated embodiment, the cylinder 151 is partially oil or other liquid filledjas indicated at 197, the other cylinder 153 containing air only, and a conduit 179a extends in the cylinder 151 from adjacent its bottom to a location at which it connects to an external conduit 17% leading to a fitting 1790, through which liquid is admitted to the tensioning cylinder body 145 below the piston 177. If desired, both cylinders 151 and l 152 may be similarly adaptecl'to contain a lower body of liquid, or a gas only system may be employed.

- .In either event, the gasor air in cylinders 151 and 152 is employed to maintain tension on the line GL,

pressure for maintaining the tension applies a tensile force on the cylinders offsetting the compressive forces to an extent determined by the pressures and cylinder dimensions. In any event, however, the structure is durable and stable, but maybe relatively compact as compared with tensioning deyicesi wherein the accumulator cylinders are non-structural and subjected to wholly unbalanced forces. In addition, the self-aligning structural connection between the tensioning cylinder and the accumulator cylinders assures alignment of the tensioning rod and cylinder with the line of force applied by the block system, so that binding is prevented and seal life is enhanced.

I claim:

1. In tensioning apparatus for maintaining tension on a line: a support adapted to be fixed to a floating platform block means secured to said support, elongate accumulator cylinder means fixed to and carried by said support, traveling block means, both of said block means being adapted to receive a line to be tensioned, elongate tensioning cylinder means including a cylinder and piston, one of said cylinder and piston being carried by said accumulator cylinder means to be supported thereby, whereby longitudinal load applied to said tensioning cylinder means is transmitted to and places said accumulator cylinder means in longitudinal compression, the other of said cylinder and piston being connected to said traveling block means for movement therewith, and means forming a passageway between said accumulator cylinder means and said tensioning cylinder means, whereby fluid displaced from said tensioning cylinder means by said piston is transferred to said accumulator cylinder means for biasing said traveling block to tension said line.

2. In tensioning apparatus as defined in claim 1, means providing a pivotal connection between said one of said cylinder and piston and said accumulator cylinder means for movement to align said tensioning cylinder means with the line of force applied to said traveling block by said line.

3. 1n tensioning apparatus as defined in claim 1, said accumulator cylinder means having a floating piston therein, and including a body of liquid at one side of said floating piston and in said tensioning cylinder.

4. In tensioning apparatus as defined in claim 1, said accumulator cylinder means having a floating piston therein, and including a body of liquid at one side of said floating piston and in said tensioning cylinder, and means for supplying gas under pressure to said accumulator cylinder means at the other side of said floating piston.

5. In tensioning apparatus as defined in claim 1, means for supplying gas under pressure to said accumulator cylinder means.

6. In tensioning apparatus as defined in claim 1, said means forming a passageway between said tensioning cylinder and said accumulator cylinder means including a conduit extending downwardly to a substantial extent into said accumulator cylinder means, and including a body of liquid in said accumulator cylinder means into which said conduit extends, said liquid surrounding said conduit.

7. In tensioning apparatus as defined in claim 1, said means forming a passageway between said tensioning cylinder and said accumulator cylinder means including a conduit extending downwardly to a substantial extent into said accumulator cylinder means, and including a body of liquid in said accumulator cylinder means in the lower portion of said accumulator cylinder means and into which said conduit extends, and means for supplying gas under pressure to said accumulator cylinder means in the upper portion of said accumulator cylinder means for exerting pressure on said liquid.

8. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block.

9. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block, and sealing means at said end of said tensioning cylinder engaged with said rod.

10. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block, and sealing means at said end of said tensioning cylinder engaged with said rod, and including means interconnecting said tensioning cylinder with said accumulator cylinder means for movement to align said tensioning cylinder and rod with the line of force applied to said traveling block by said line.

11. In tensioning apparatus for maintaining tension on a line: support means adapted to be fixed to a floating platform, a pair of elongated accumulator cylinders fixed to and carried by said support means in side-byside relation, an elongated tensioning cylinder, means carrying said tensioning cylinder by said accumulator cylinders in centralized and parallel relation to be supported thereby, whereby longitudinal load applied to said tensioning cylinder is transmitted to and places said accumulator cylinders in longitudinal compres' sion, said tensioning cylinder having a piston and a piston rod projecting from an end of said tensioning cylinder between said accumulator cylinders, traveling block means connected to said rod, stationary block means carried by said support means, said block means being adapted to receive a line to be tensioned, and means providing fluid communication between said tensioning cylinder at its rod end and at least one of said accumulator cylinders.

12. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line.

13. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including a yoke extending about said tensioning cylinder and pivotally connected to each of said accumulator cylinders, and trunnion means supported on said yoke and engaged with said tensioning cylinder.

14. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including means forming a universal connection between said accumulator cylinders and said tensioning cylinder.

15. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including a surrport plate between said accumulator cylinders and a collar on said tensioning cylinder, said plate and said collar having confronting faces forming a ball and socket type support.

16. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, said tensioning cylinder having at its rod end a bore, and a sealing sleeve in said bore surrounding said rod in sealing relation therewith.

17. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, said tensioning cylinder having at its rod end a bore, and a sealing sleeve in said bore surrounding said rod in sealing relation therewith, and including releasable connecting means securing said traveling block means to the end of said rod and removable therefrom to enable said. sealing sleeve to be removed endwise from said rod.

18. In tensioning apparatus as defined in claim 11, said accumulator cylinders each having a floating piston therein, and including a body of liquid filling said tensioning cylinder and accumulator cylinders between the respective floating pistons and said tensioning cylinder piston, and said accumulator cylinders being gas filled at the other side of said floating pistons from said body of liquid.

19. In tensioning apparatus as defined in claim 11, said means providing communication between said tensioning cylinder and said accumulator cylinders including a conduit extending downwardly into an opening in one of said accumulator cylinders, said one of said accumulator cylinders having therein a body of liquid partially filling the same and forming a portion of a quantity of liquid filling said conduit and said tensioning cylinder below said piston.

20. In tensioning apparatus as defined in claim 11, means for supplying gas under pressure to said accumulator cylinders to establish the tension on said line.

21. In tensioning apparatus as defined in claim 11, said fluid communication means containing a liquid in said rod end of said tensioning cylinder and in a portion of said one of said accumulator cylinders, and means providing gas under pressure in the other of said accumulator cylinders and in another portion of said one of said accumulator cylinders, said gas being permitted to flow between said accumulator cylinders, said other of said accumulator cylinders containing gas only.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO: 3,897,045 DATED July 29, 1975 INVENTOR(S) BRYAN V. BUTLER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 29: after "away" insert a Column 3, line 56: after "block" insert --sections--.

Column 4, line 1: cancel the (second occurrence).

Signed and Scaled this thirtieth Day of December1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner album: and Trademarks 

1. In tensioning apparatus for maintaining tension on a line: a support adapted to be fixed to a floating platform block means secured to said support, elongate accumulator cylinder means fixed to and carried by said support, traveling block means, both of said block means being adapted to receive a line to be tensioned, elongate tensioning cylinder means including a cylinder and piston, one of said cylinder and piston being carried by said accumulator cylinder means to be supported thereby, whereby longitudinal load applied to said tensioning cylinder means is transmitted to and places said accumulator cylinder means in longitudinal compression, the other of said cylinder and piston being connected to said traveling block means for movement therewith, and means forming a passageway between said accumulator cylinder means and said tensioning cylinder means, whereby fluid displaced from said tensioning cylinder means by said piston is transferred to said accumulator cylinder means for biasing said traveling block to tension sAid line.
 2. In tensioning apparatus as defined in claim 1, means providing a pivotal connection between said one of said cylinder and piston and said accumulator cylinder means for movement to align said tensioning cylinder means with the line of force applied to said traveling block by said line.
 3. In tensioning apparatus as defined in claim 1, said accumulator cylinder means having a floating piston therein, and including a body of liquid at one side of said floating piston and in said tensioning cylinder.
 4. In tensioning apparatus as defined in claim 1, said accumulator cylinder means having a floating piston therein, and including a body of liquid at one side of said floating piston and in said tensioning cylinder, and means for supplying gas under pressure to said accumulator cylinder means at the other side of said floating piston.
 5. In tensioning apparatus as defined in claim 1, means for supplying gas under pressure to said accumulator cylinder means.
 6. In tensioning apparatus as defined in claim 1, said means forming a passageway between said tensioning cylinder and said accumulator cylinder means including a conduit extending downwardly to a substantial extent into said accumulator cylinder means, and including a body of liquid in said accumulator cylinder means into which said conduit extends, said liquid surrounding said conduit.
 7. In tensioning apparatus as defined in claim 1, said means forming a passageway between said tensioning cylinder and said accumulator cylinder means including a conduit extending downwardly to a substantial extent into said accumulator cylinder means, and including a body of liquid in said accumulator cylinder means in the lower portion of said accumulator cylinder means and into which said conduit extends, and means for supplying gas under pressure to said accumulator cylinder means in the upper portion of said accumulator cylinder means for exerting pressure on said liquid.
 8. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block.
 9. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block, and sealing means at said end of said tensioning cylinder engaged with said rod.
 10. In tensioning apparatus as defined in claim 1, said piston having a rod extending from an end of said tensioning cylinder and connected to said traveling block, and sealing means at said end of said tensioning cylinder engaged with said rod, and including means interconnecting said tensioning cylinder with said accumulator cylinder means for movement to align said tensioning cylinder and rod with the line of force applied to said traveling block by said line.
 11. In tensioning apparatus for maintaining tension on a line: support means adapted to be fixed to a floating platform, a pair of elongated accumulator cylinders fixed to and carried by said support means in side-by-side relation, an elongated tensioning cylinder, means carrying said tensioning cylinder by said accumulator cylinders in centralized and parallel relation to be supported thereby, whereby longitudinal load applied to said tensioning cylinder is transmitted to and places said accumulator cylinders in longitudinal compression, said tensioning cylinder having a piston and a piston rod projecting from an end of said tensioning cylinder between said accumulator cylinders, traveling block means connected to said rod, stationary block means carried by said support means, said block means being adapted to receive a line to be tensioned, and means providing fluid communication between said tensioning cylinder at its rod end and at least one of said accumulator cylinders.
 12. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment oF said tensioning cylinder and rod with the line of force applied to said traveling block by said line.
 13. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including a yoke extending about said tensioning cylinder and pivotally connected to each of said accumulator cylinders, and trunnion means supported on said yoke and engaged with said tensioning cylinder.
 14. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including means forming a universal connection between said accumulator cylinders and said tensioning cylinder.
 15. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, including a surrport plate between said accumulator cylinders and a collar on said tensioning cylinder, said plate and said collar having confronting faces forming a ball and socket type support.
 16. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, said tensioning cylinder having at its rod end a bore, and a sealing sleeve in said bore surrounding said rod in sealing relation therewith.
 17. In tensioning apparatus as defined in claim 11, said means connecting said tensioning cylinder to said accumulator cylinders comprising means enabling alignment of said tensioning cylinder and rod with the line of force applied to said traveling block by said line, said tensioning cylinder having at its rod end a bore, and a sealing sleeve in said bore surrounding said rod in sealing relation therewith, and including releasable connecting means securing said traveling block means to the end of said rod and removable therefrom to enable said sealing sleeve to be removed endwise from said rod.
 18. In tensioning apparatus as defined in claim 11, said accumulator cylinders each having a floating piston therein, and including a body of liquid filling said tensioning cylinder and accumulator cylinders between the respective floating pistons and said tensioning cylinder piston, and said accumulator cylinders being gas filled at the other side of said floating pistons from said body of liquid.
 19. In tensioning apparatus as defined in claim 11, said means providing communication between said tensioning cylinder and said accumulator cylinders including a conduit extending downwardly into an opening in one of said accumulator cylinders, said one of said accumulator cylinders having therein a body of liquid partially filling the same and forming a portion of a quantity of liquid filling said conduit and said tensioning cylinder below said piston.
 20. In tensioning apparatus as defined in claim 11, means for supplying gas under pressure to said accumulator cylinders to establish the tension on said line.
 21. In tensioning apparatus as defined in claim 11, said fluid communication means containing a liquid in said rod end of said tensioning cylinder and in a portion of said one of said accumulator cylinders, and means providing gas under pressure in the other of said accumulator cylinders and in another portion of said one of said accumulator cylinders, said gas being permitted to flow between said accumulator cylinders, said other of said accumulator cylinders containing gas only. 